The electrical effects of injury at various myocardial locations

1. 1. When an exploring electrode was placed on an area of subepicardial injury (the indifferent electrode being on the left hind paw), an upward S-T displacement occurred; a downward S-T displacement was found when the exploring electrode was placed on the marginal zone surrounding the area of injury. When a contact wick exploring electrode was moved from the area of injury to surrounding uninvolved tissue and continuous records made, a narrow transitional zone was found in which the S-T segment was isoelectric. In regions removed from the area of injury and exploring electrode showed the S-T segment to be slightly depressed or isoelectric in the surrounding epicardium on the same side as the subepicardial injury, depending upon the extent of the injury. When the exploring electrode was on the epicardium of the wall opposite to that injured, downward S-T displacement occurred which was maximal when the exploring electrode was on the axis normal to and intersecting the center of the plane of the area of injury. In passing from the wall injured to the opposite wall a zone in which the S-T segment was isoelectric was found. 2. 2. While the area of upward S-T displacement was confined to the area of injury at the time of injury, with the passage of time S-T elevation extended to surrounding regions on the same wall. At all times, however, the S-T segment displacement was greatest over the area of epicardial injury. The spread of injury effects was uneven. The S-T displacement in equidistant points from the injured area were not of the same magnitude nor did they appear at the same time. The injury effects were usually greater in areas toward the base of the heart. 3. 3. The electrical effects produced by subendocardial pressure or scrape injury on endocardial or intracavity exploring electrodes were similar to those produced over the epicardial surface of a subepicardial injury. These included S-T elevation and T-R depression; monophasic ventricular curves were found. Continuous records over the subendocardial injury, made while the injury was produced, revealed three types of injury effects on the S-T and T-R segments: (1) downward displacement of T-R with no or slight upward displacement of S-T, that is, primarily an injury current of rest; (2) slight downward displacement of T-R and marked upward displacement of S-T, that is, primarily an injury current of activity; and (3) most commonly, almost equal downward displacement of T-R and upward displacement of